XVI. STABLE ISOTOPES AS TRACERS 583 



compute the proper setting of the other variable for the selected 

 particle by means of equation (2). The final searching, of course, is 

 done experimentally. Peaks corresponding to water vapor and car- 

 bon monoxide are helpful here; they can always be detected even 

 with thoroughly degassed apparatus. 



When measurements of isotopic ratios are made with a nonrccord- 

 ing mass spectrometer, the plotted observations have an appearance 

 similar to the experimental curve for a carbon dioxide analysis re- 

 produced in Figure 4. In practice, observations are required on the 

 top of the peaks only, except for just enough readings elsewhere to 

 enable corrections to be made for the background. To avoid errors 

 that may arise from any instability in the electrical circuits or from 

 gas pressure variation in the ion source, a complete set of data should 

 include perhaps a dozen observations. These should be taken alter- 

 nately on each of the peaks and within the shortest time possible. 

 Hence the electrical circuits should permit an almost instantaneous 

 shift from one peak to another (see 5). Multiple sets of control 

 equipment, one for each peak, are required in which provision has 

 been made for adjusting not only the proper ion-accelerating voltage 

 corresponding to each peak, but also the correct sensitivity of ampli- 

 fier H (see Fig. 2). The latter may involve shifting from one input 

 resistor, R, to another. 



6. Calculation of Results 



Typical experimental observations for a C^^ determination with a 

 carbon dioxide test sample are shown graphically in Figure 4. Ion 

 current readings cover mass numbers 44, 45, and 46. The noticeable 

 background deflection that appears in this figure must be subtracted 

 from the various peak heights before their ratios are computed. 

 Further, the 45 peak has received a small contribution to its height 

 from the presence of C^^O^^O^^ molecules in addition to the desired 

 Q13Q16Q16 molecules. This constant 0^^ correction amounts to 0.08% 

 and may be made either before or after the calculation of the 44/45 

 ratio. Assuming that the two corrections just mentioned have been 

 made and that the ratio, \\R, of the resultant ion currents at mass 44 

 to that at mass 45 has been calculated, then the concentration of C'^ 

 in atom per cent is given by : 



(C'')100 100 ^ ^ ,„, 



